With the upcoming release of PowerShell v2, v1 material will soon be out of date... but not completely. This online book (also available in PDF) is on v1, and is a great way to get you up to speed before RTM of Windows 7 and Server 2008 R2, wher PowerShell v2 is built-in. It is by Dr. Tobias Weltner, MS MVP for PowerShell.